Three electrically heated triangular ducts were used to simulate the thermal behaviours of turbulent air-flows through triangular passages in compact heat-exchangers (Fig. 1). Three sharp-cornered isosceles-triangular duralumin ducts were fabricated, each of the same length of 2.4 m and hydraulic diameter of 0.44 m, but with three different apex-angles, namely 40°, 60° and 90°. The experimental investigations were performed over a wide range of Reynolds number (Re), namely 5000 < ReD < 20000. Non-dimensional correlations were deduced from the measurements to enable the average convective heat-transfer coefficient for convection from the walls of the isosceles-triangular duct (of specified apex-angle) to the turbulent air to be predicted. All other factors remaining invariant, in order to achieve the maximum rate of heat transfer, the apex-angle of the isosceles triangle should be 60°, i.e. the duct's cross-section should be equilateral. A parameter known as the 'total reciprocal-angle', ΦT. is defined in order to facilitate analysing the effect of corner geometry on the forced-convection occurring inside isosceles-triangular-sectioned ducts.